Generally, vaccines have proven to be successful methods for the prevention of infectious diseases. Generally, they are cost effective, and do not induce antibiotic resistance to the target pathogen or affect normal flora present in the host. In many cases, such as when inducing anti-viral immunity, vaccines can prevent a disease for which there are no viable curative or ameliorative treatments available.
Vaccines function by triggering the immune system to mount a response to an agent, or antigen, typically an infectious organism or a portion thereof that is introduced into the body in a non-infectious or non-pathogenic form. Once the immune system has been “primed” or sensitized to the organism, later exposure of the immune system to this organism as an infectious pathogen results in a rapid and robust immune response that destroys the pathogen before it can multiply and infect enough cells in the host organism to cause disease symptoms.
The agent, or antigen, used to prime the immune system can be the entire organism in a less infectious state, known as an attenuated organism, or in some cases, components of the organism such as carbohydrates, proteins or peptides representing various structural components of the organism.
In many cases, it is necessary to enhance the immune response to the antigens present in a vaccine in order to stimulate the immune system to a sufficient extent to make a vaccine effective, i.e., to confer immunity. Many protein and most peptide and carbohydrate antigens, administered alone, do not elicit a sufficient antibody response to confer immunity. Such antigens need to be presented to the immune system in such a way that they will be recognized as foreign and will elicit an immune response. To this end, additives (adjuvants) have been devised which immobilize antigens and stimulate the immune response.
The best known adjuvant, Freund's complete adjuvant, consists of a mixture of mycobacteria in an oil/water emulsion. Freund's adjuvant works in two ways: first, by enhancing cell and humoral-mediated immunity, and second, by blocking rapid dispersal of the antigen challenge (the “depot effect”). However, due to frequent toxic physiological and immunological reactions to this material, Freund's adjuvant cannot be used in humans.
Another molecule that has been shown to have immunostimulatory or adjuvant activity is endotoxin, also known as lipopolysaccharide (LPS). LPS stimulates the immune system by triggering an “innate” immune response—a response that has evolved to enable an organism to recognize endotoxin (and the invading bacteria of which it is a component) without the need for the organism to have been previously exposed. While LPS is too toxic to be a viable adjuvant, molecules that are structurally related to endotoxin, such as monophosphoryl lipid A (“MPL”) are being tested as adjuvants in clinical trials. Both LPS and MPL have been demonstrated to be agonists to the human toll-like receptor-4 (TLR-4). Currently, however, the only FDA-approved adjuvant for use in humans is aluminum salts (Alum) which are used to “depot” antigens by precipitation of the antigens. Alum also stimulates the immune response to antigens.
Accordingly, there is a need to develop synthetic methods for preparing compounds which can be co-administered with antigens in order to stimulate the immune system to generate a more robust antibody response to the antigen than would be seen if the antigen were injected alone or with Alum.